1. Field of the Invention
The present invention relates to an electrical charger for use in an image formation apparatus such as an electro-photographic printer, facsimile, or a copying apparatus.
2. Description of the Related Art
Conventionally, the image formation apparatus of an electro-photographic printer and the like a corona discharge device for charging a surface of an image carrier such as a photo-sensitive body in a process cartridge of the electro-photographic printer. The corona discharge device is advantageous for uniformly charging the surface of the image carrier up to a predetermined potential.
Alternatively, a brush electrical charger can also used. In the brush electrical charger, a charging member is brought into direct contact with the photo-sensitive body to be charged. A roller charger and a film charger are further alternative types of contact type electrical charges.
Here, the voltage applied to the contact type chargers may be only DC voltage or DC voltage in combination with AC voltage.
However, the aforementioned conventional chargers have various problems. In the case of a charger using the corona discharge, there is a problem that ozone is generated around the charger. From the viewpoint of the boundary, it is preferable to suppress the ozone generation to a minimum.
On the other hand, in the case of a contact type charger that can suppress ozone generation, application of DC voltage lowers the charging uniformity in comparison to the corona discharge type charger. Especially in the high resolution range of 600 dpi (dots per inch) or above, fluctuations in resistance as well as in the contact state prevent to obtain a uniform charging, thereby causing undesired lines in a half-tone image.
Moreover, in case of chargers using DC voltage in combination with AC voltage, the charge uniformity is improved in comparison to the case applying only DC voltage. However, if the frequency of the AC voltage is too low, a Moire image is generated. To avoid this, it is necessary to set the AC voltage frequency at a sufficiently high value. More specifically, the AC voltage frequency should be set at a value greater than the number of dots made to pass by rotation of the photosensitive body through a particular position of the charger per unit time, so that charging is carried out in a high frequency region. However, when such a high frequency AC voltage is applied, there is a problem that the charger causes a noise due to vibration.
Japanese Patent Publication (unexamined) A-7-98534 discloses as means for suppressing charge nonuniformity, an AC voltage frequency F defined by a formula as follows: F.gtoreq.R.times.S, wherein R represents the resolution (dots/inch) and S represents the photo-sensitive body speed (inch/second). The frequency defined by this formula becomes a high frequency wave in case of high resolution, increasing the aforementioned problem of noise.
Japanese Patent Publication (unexamined) A-7-281503 discloses a brush type charging apparatus capable of eliminating a stripe-shaped black paint at a position corresponding to a pinhole present on a photo-sensitive body or a brush trace due to a uniform potential on the photo-sensitive body in half-tone printing.